1. Field of the Invention
The present invention relates to gelled fracturing fluids of the type used in well bore operations and particularly to a method for producing a gradual reduction in the viscosity of a gelled fracturing fluid through the use of enzymes incorporated in the gelled fluid which are active over broad pH and temperature ranges.
2. Description of the Prior Art
During hydraulic fracturing, a sand laden fluid is injected into a well bore under high pressure. Once the natural reservoir pressures are exceeded, the fracturing fluid initiates a fracture in the formation which generally continues to grow during pumping. The treatment design generally requires the fluid to reach maximum viscosity as it enters the fracture which affects the fracture length and width. This viscosity is normally obtained by the gellation of suitable polymers, such as a suitable polysaccharide. The gelled fluid can be accompanied by a propping agent which results in placement of the propping agent within the fracture thus produced. The proppant remains in the produced fracture to prevent the complete closure of the fracture and to form a conductive channel extending from the well bore into the formation being treated once the fracturing fluid is recovered.
The recovery of the fracturing fluid is accomplished by reducing the viscosity of the fluid to a low value such that it flows naturally from the formation under the influence of formation fluids and pressure. This viscosity reduction or conversion is referred to as "breaking" and can be accomplished by incorporating chemical agents, referred to as breakers, into the initial gel.
Historically, the application of breakers in fracturing fluids at elevated temperatures, i.e., above about 170-190 degrees F., has been a compromise between maintaining proppant transport and achieving the desired fracture conductivity. Conventional oxidative breakers react rapidly at elevated temperatures, potentially leading to catastrophic loss of proppant transport. Encapsulated oxidative breakers have experienced limited utility at elevated temperatures due to a tendency to release prematurely or to have been rendered ineffective through payload self-degradation prior to release.
Enzymes, from a theoretical perspective, are known to provide superior performance relative to oxidative breakers. This is due to the inherent specificity and the infinite polymer degrading activity of enzymes. However, the application of enzymes has historically been limited to low-temperature fracturing treatments due to the perceived pH and temperature constraints of these breaker systems.
The application of effective enzyme breaker technology to high temperature fracturing treatments should result in improved well productivity relative to treatments utilizing conventional breaker technology. One area of particular improvement should be in reducing polymeric damage caused by polymeric filter cake buildup or unbroken gel residue. A polymeric filter cake is a dense mass of polymer deposited on the formation face by dynamic fluid loss while pumping and/or concentrated within the proppant-pack by fracture width reduction upon fracture closure. The polymers used as gelling agents in fracturing treatments are too large to penetrate the rock matrix and are, therefore, concentrated within the fracture. Several studies have documented that the polymer concentration within the fracture is as much as 20-fold the surface gelling agent concentration.
Polymeric damage in the form of unbroken gel residue or dynamically formed filter cake can significantly reduce well productivity. For example, gel residue damage can be characterized as the blocking of pore throats by unbroken viscous gel having limited mobility or, by insoluble polymer fragments. The degree of damage is proportional to the amount of fracture pore volume occupied by the gel residue. The use of enzyme breakers allows the degradation of the polymeric gelling agents in the fluid to proceed in controllable manner to reduce the fracturing fluid viscosity by cleavage of the polymer backbone into fragments which will remain soluble in the aqueous base fluid. The advantageous use of enzyme breakers in high temperature applications would reduce polymeric damage through minimization of the amount of gel residue remaining in the fracture after load recovery.
In addition to the importance of providing a breaking mechanism for the gelled fluid to facilitate recovery of the fluid and to optimize fracture conductivity by minimizing polymeric damage, the timing of the break is also of great importance. Gels which break prematurely can cause suspended proppant material to settle out of the gel before being introduced a sufficient distance into the produced fracture. Premature breaking can also result in a premature reduction in the fluid viscosity resulting in a less than desirable fracture length in the fracture being created.
On the other hand, gelled fluids which break too slowly can cause slow recovery of the fracturing fluid from the produced fracture with attendant delay in resuming the production of formation fluids. Additional problems can result, such as the tendency of proppant to become dislodged from the fracture, resulting in at least partial closing and decreased efficiency of the fracturing operation.
For purposes of the present application, premature breaking will be understood to mean that the gel viscosity becomes diminished to an undesirable extent before all of the fluid is introduced into the formation to be fractured.
Optimally, the fracturing gel will begin to break when the pumping operations are concluded. For practical purposes, the gel should be completely broken within a specific period of time after completion of the fracturing period. At higher temperatures, for example, about 24 hours is sufficient. A completely broken gel will be taken to mean one that can be flushed from the formation by the flowing formation fluids or that can be recovered by a swabbing operation. In the laboratory setting, a completely broken, non-crosslinked gel is one whose viscosity is either about 10 centipoises or less as measured on a Model 50 Fann viscometer R1/B1 at 300 rpm or less than 100 centipoises by Brookfield viscometer spindle #1 at 0.3 rpm.
Obtaining controlled breaks using various prior art chemical agents, both oxidants and enzymes, has proved difficult. Common oxidants do not break the polysaccharide backbone into monosaccharide units. The breaks are nonspecific, creating a mixture of macromolecules. Further, common oxidants are difficult to control. They react with things other than the polymeric gel. Oxidants can react, for example, with the tubing and linings used in the oil industry as well as resins on resin coated proppants.
Using enzymes for controlled breaks circumvents the above noted oxidant problems. Conventional enzyme breaker systems generally degrade the gel polymers inadequately, however. These enzymes, for example, the cellulases, hemi-cellulases, amylases, pectinases, and their mixtures are familiar to those in the well service industry. These enzymes break the bonds that connect the monosaccharides into a polysaccharide backbone, for instance, the 1,4-.alpha.-D-galactosiduronic linkages in pectin. These conventional enzyme breaker systems are nonspecific and cause random breaks. As a result, these prior art enzyme systems only partially degrade the polysaccharide polymer. Instead of fragmenting almost completely into much smaller fragments such as monosaccharides, the enzymes break the polysaccharide gel into larger fragments consisting of a mixture of disaccharides, oligosaccharides and polysaccharides. These larger gel fragments have been shown to cause residue problems in the fractured formation once the fracturing operation is complete. Such residue decreases productivity by restricting the flow of fluid and plugging the formation.
The present invention has as its object to provide a break mechanism for a gelled fracturing fluid which yields high initial viscosity with little change during pumping but which produces a rapid break in the gel after pumping is completed to allow immediate recovery of the fluid from the formation.
Another object of the invention is to provide a gel system for a well fracturing operation which can break the gel polymers within a wide range of pH and temperature without interfering with the crosslinking chemistry.
Another object of the invention is to provide an enzyme breaker system for a gelled fracturing fluid which produces a controlled break over pH range from about 3 to 11 and at temperatures in the range from about 60 to 300 degrees F., or more, and which decreases the amount and size of residue left in the formation after recovery of the fluid from the formation.
Another object of the invention is to provide a thermo-stable, polymer linkage specific enzyme breaker which is catalytically active and temperature stable in the range from about 60 to 300 degrees F. and at pH's between about 3.0 to 11.0 and which decreases the amount of residue left in the formation after recovery of the fluid from the formation.